[ES6] Worker should support ES6 modules

[Yusuke Suzuki]

...

[Saleh Abdel Motaal]

It seems that support `new Worker(…, {type: "module"})` is limited or missing.

In a project that uses only pure ES modules served using a very basic express server (ie express.static) over http://127.0.0.1:8080 or http://localhost:8080 a worker module with static import statements:

```
import {…} from '…'
```

Throws: 
  SyntaxError: Unexpected token '{'. import call expects exactly one argument.

It should be noted that those static import statements are synthesized using rollup and the resulting files sometimes include one or more variable declarations before the occurrence of the import statements.

The same project does not throw in Chrome.

[Guy Bedford]

Note that supporting dynamic `import()` in Web Workers can also be a first start along these lines.